1. Field of the Invention
This invention relates to a method for planarizing the surface of ceramic substrates, in particular to vapor deposition of silicon nitride onto the surface of ceramic substrates; and to the planarized surface resulting from such deposition of silicon nitride.
2. Description of the Prior Art
Silicon nitride has been deposited onto semiconductor substrates and connection metallurgy to form conformal insulating layers. For example, U.S. Pat. No. 4,423,547 of Farrar et al. and assigned to the assignee of the present invention teaches a method for providing high density multiple level metallurgy for integrated circuit devices by plasma spraying a thin layer of silicon nitride over a layer of interconnection metallurgy. The '547 patent teaches a method for forming a passivating layer of silicon nitride. Vias can be etched in the silicon nitride layer and then a thick layer of a polyimide deposited. The deposition process of the '547 patent produces a conformal silicon nitride layer and does not planarize the substrate.
Planarizing the surface of ceramic substrates has been done by mechanical methods including grinding, lapping and polishing. These methods are time consuming, expensive, and in addition can not readily be done on selected regions of the substrate.
Glazing, which has been used to produce a planarized surface, like the mechanical methods listed above, is suited for planarizing the entire surface. Glasses used for glazing have low melting points and generally have elements added to depress the melting point such as calcium, sodium and potassium. These added elements are required to bring the melting point of the glaze below the melting point of the substrate. The glasses may also contain impurities such as lead, uranium and thorium. Since uranium and thorium are alpha emitters the glazing materials may be a source of alpha particles. In addition, glazing tends to form a surface with long wavelength variation and the edges of the glaze tend to round due to surface tension. Thus the usable planarized area resulting from glazing is limited. An additional disadvantage of glazes is that because of the brittle nature of the glasses used for glazing small cracks tend to propagate readily when the glazed surface is subject to stress such as may be encountered during fabrication. In addition the glazing process is inherently unsuitable for subsequent selective removal of the glaze. The glazing process is also difficult to control and extremely thick, in the order of 1 or more mils.